Continent-Ocean Interaction: Role of Weathering

Size: px

Start display at page:

Download "Continent-Ocean Interaction: Role of Weathering"

Chester Russell
5 years ago
Views:

1 Institute of Astrophysics and Geophysics (Build. B5c) Room 0/13 Phone: th February 2018 Organisation of the Lecture 1 Carbon cycle processes time scales modelling: why? 2 Model development: general principles 3 Illustration: simple carbon cycle model 4 Conclusions and outlook

2 Carbon Cycle: Processes and Time Scales ATMOSPHERE deforestation assimilation respiration assimilation water exchange Dissolved inorganic carbon Dissolved organic carbon Particulate organic carbon Mantle degassing air sea exchange Marine biota weathering surface water respiration particulate flux OCEAN deep water Seafloor alteration BIOSPHERE riverborne material sedimentation (organic & inorganic) litterfall Litter Humus Peat Carbonate Coal, oil, gas Carbonate minerals Dispersed carbon fossil fuel burning LITHOSPHERE volcanism (Adapted from Holmén, 1992) Natural Processes with long time scales Natural Processes with short time scales Human Perturbations Modelling OBSERVATION (Data) HYPOTHESES (Theory) STATISTICAL LAWS MECHANISTIC LAWS CALIBRATION VALIDATION MODEL - statistical - mechanistic DIAGNOSTIC USAGE (comprehension) PROGNOSTIC USAGE (prediction)

3 Model Development: General Principles Four stages 1 Problem Identification 2 Model Formulation 3 Model Solution 4 Interpretation of the results Equal importance for each stage Not a uni-directional procedure (following Boudreau, 1997) Development of a Model Formulation processes to include / exclude mathematical representation of the processes approximations adopted hypotheses made Solution depends on the situation Interpretation secondary results: consequences model to be refined or to simplified (following Boudreau, 1997)

4 Illustration: Application to an Actual Question Question How much CO 2 is released by volcanic and hydrothermal activity (metamorphic fluxes included)? How does this compare to the amount of CO 2 released by human activity? Model Formulation: Hypotheses and Simplifications Time Scale: 1,000 10,000 years and more little variability of volcanic and hydrothermal fluxes biosphere at steady state : fluxes have no influence burial of organic matter counter-balanced by kerogen carbon weathering: fluxes cancel out sea-floor weathering poorly known and small: neglected Steady state

5 Carbon Cycle Model: Processes Considered volcanic output C vol 01 silicate weathering C sil a Exchange of carbon between ocean and atmosphere Exchange of carbon between ocean/atmosphere and crust 01 ATMOSPHERE carbonate deposition air sea exchange C o a C a o Ccar a OCEAN carbonate weathering C car r C sed C hyd CRUST hydrothermal processes Carbonate Chemistry in Seawater Carbonate system equilibria CO 2(aq) + 2 H 2 O HCO 3 + H 3O + HCO 3 + H 2O CO H 3O + Special roles played by particular species atmospheric p CO2 [CO 2(aq) ] surface CaCO 3 burial [CO 2 3 ] deep sea Speciation calculated from combinations Dissolved Inorganic Carbon C T = [CO 2(aq) ] + [HCO 3 ] + [CO2 3 ] Total Alkalinity A T [HCO 3 ] + 2[CO2 3 ] + [B(OH) 4 ] + [OH ] [H 3 O + ]

6 Carbon Cycle Model: Fluxes Considered volcanic output C vol 01 C sil a C sed silicate weathering Creation and destruction of alkalinity Exchange of carbon between ocean and atmosphere Exchange of carbon between ocean/atmosphere and crust A sil ATMOSPHERE carbonate deposition air sea exchange C o a C a o Ccar a A car OCEAN carbonate weathering C car r C hyd A sed CRUST hydrothermal processes Carbon Cycle Model: Conservation Equations C atm : total amount of C in the atmosphere C oce : total amount of C in the ocean C atm + C oce = C A : total amount of alkalinity in the ocean dc atm dc oce = C vol C sil a C car a + C o a C a o = C hyd + C sil a + C car a + C car r C o a + C a o C sed dc atm + dc oce = dc da = C hyd + C vol + C car r C sed = A sil + A car A sed

7 Typical Weathering Reactions for Silicate Minerals Dissolution of albite with precipitation of kaolinite 2NaAlSi 3 O 8 + 2CO H 2 O Al 2 Si 2 O 5 (OH) 4 + 2Na + + 2HCO 3 + 4H 4 SiO 4 Dissolution of anorthite with precipitation of kaolinite CaAl 2 Si 2 O 8 + 2CO 2 + 3H 2 O Al 2 Si 2 O 5 (OH) 4 + Ca HCO 3 Dissolution of microcline with precipitation of pyrophillite 2KAlSi 3 O 8 + 2CO 2 + 6H 2 O Al 2 Si 4 O 10 (OH) 2 + 2K + + 2HCO 3 + 2H 4 SiO 4 Typical Weathering Reactions for Silicate Minerals Dissolution of chlorite with precipitation of kaolinite Mg 5 Al 2 Si 3 O CO 2 + 5H 2 O Al 2 Si 2 O 5 (OH) 4 + 5Mg HCO 3 + H 4SiO 4 Dissolution of microcline with precipitation of gibbsite KAlSi 3 O 8 + CO 2 + 4H 2 O Al(OH) 3 + K + + HCO 3 + H 4SiO 4

8 Sources and Sinks of DIC and TA in the Ocean Sources : continental weathering carbonate rocks: congruent dissolution CaCO 3 + CO 2 + H 2 O Ca HCO 3 silicate rocks: incongruent dissolution silicate mineral + b CO 2 + water secondary minerals + cations + b HCO 3 + s H 4SiO 4 Sinks : burial of biogenic carbonates Ca HCO 3 CaCO 3 + CO 2 + H 2 O Cycles to the Carbon Cycle: Ca, K, Mg, Na, Si Residence times of coupled cycles elements in the oceans Element τ oc Note (10 6 yr) Ca 1 Mg 13 K 12 Na 83 Si 0.02 DIC 0.07 org. and inorg. sinks 0.10 inorg. sinks only Alk 0.05

9 Carbon Cycle Silicate Cycle Coupling Geochemical carbonate and silicate cycles Simplification: neglect K and Na contributions No significant K- or Na-carbonate depositions Only 5% of the total riverine HCO 3 flux provided by Na- and K-silicate dissolution (Berner, 2004, based upon Berner and Berner, 1996) According to Gaillardet et al. (1999), this fraction is 19%, to be compared with 21% from Ca- and Mg-silicates This oceanic HCO 3 source is counterbalanced by the HCO 3 sink represented by authigenic Na- and K-mineral precipitation in marine sediments (reverse weathering, very slow process): 2K + + 3Al 2 Si 2 O 5 (OH) 4 + 2HCO 3 2KAl 3 Si 3 O 10 (OH) 2 + 5H 2 O + CO 2 Carbon Cycle Silicate Cycle Coupling Geochemical carbonate and silicate cycles Weathering of Ca- (or Mg-) carbonate CaCO 3 + CO 2 + H 2 O Ca HCO 3 Precipitation and sediment burial of carbonates Ca HCO 3 CaCO 3 + CO 2 + H 2 O Net balance:

10 Carbon Cycle Silicate Cycle Coupling Geochemical carbonate and silicate cycles Weathering of Ca- (or Mg-) silicate CaSiO 3 + 2CO 2 + 3H 2 O Ca 2+ + H 4 SiO 4 + 2HCO 3 Precipitation and sediment burial of carbonate and opal Ca HCO 3 CaCO 3 + CO 2 + H 2 O H 4 SiO 4 SiO 2 + 2H 2 O Net balance CaSiO 3 + CO 2 CaCO 3 + SiO 2 Combined with reverse reaction (metamorphism) CaSiO 3 + CO 2 CaCO 3 + SiO 2 Urey s Reaction Global Balance of the Ocean-Atmosphere System Relationships between carbon and alkalinity fluxes C car r = C car a A sil = C sil a A car = C car a + C car r = 2C car r A sed = 2C sed Upon introduction into the C et A balance equations: dc da = C hyd + C vol + C car r C sed = A sil + A car A sed

11 Carbon Cycle Model: Resolution dc da = C hyd + C vol + C car r C sed = C sil a + 2C car r 2C sed Carbon Cycle Model: Resolution Steady state conditions: t > 10 6 yr dc = 0 et da = 0 Accordingly, the balance equations for C et A become C hyd + C vol + C car r C sed = 0 (1) C sil a + 2C car r 2C sed = 0 (2) Finally, equation (1) 1 2 equation (2) yields C hyd + C vol = 1 2 C sil a

12 Carbon Cycle Model: Resolution Initial problem reduced to: C sil a =? 66% {}}{ C riv = C sil a +C car a }{{}}{{} 32% 34% 34% {}}{ + C car r }{{} 34% Riverine HCO 3 data analysis total amount: 31,6 37, molhco 3 66% stem from the atmosphere per year Hence: and thus C sil a = 0.32 C riv C hyd + C vol = 0.16 C riv. Solution and Interpretation Result Since we find that C riv = ( ) mol C/yr, C hyd + C vol = ( ) mol C/yr

13 Solution and Interpretation Interpretation Comparison with anthropogenic CO 2 emissions Secondary result: sedimentary flux C sed C sed = C hyd + C vol + C car r (equation (1)) = 1 2 C sil a + C car r = 1 2 C sil a C car a C car r = 1 2 C riv Hence: C sed = ( ) mol C/an Solution and Interpretation CO 2 Emissions (10 12 gc/yr) Fossil Fuel Combustion and Cement Production Oil Coal Gas Cement Production Flaring Total Emissions Year CO 2 Emissions (10 12 molc/an) Coal Oil Gas Cement Flaring Total Units: Tmol C/yr (original data in Tg C/yr). Data sources: Boden et al. (2011), for years before 2008; Boden and Blasing (2012) for (preliminary).

14 Carbon Cycle: Present-day and Pre-industrial ATMOSPHERE ~ CO 2 (preindustrial ) organic C = CO + HCO + CO OCEAN LITHOSPHERE CaCO 3 Riverine HCO 3 Weathering CO 2 surface deep Deep sea CaCO deposition 3 Volcanic CO Shallow water CaCO deposition Carbonate Rock Weathering Reservoir sizes ~ VEGETATION & SOIL in mol C Fluxes in 1012 mol C/yr Carbon Cycle: Present-day and Pre-industrial ATMOSPHERE CO 2 (preindustrial ) = CO + HCO +CO Riverine HCO 3 Weathering CO 2 surface Volcanic CO ~ deep ~ Carbonate Rock Weathering VEGETATION & SOIL Shallow water organic C CaCO 3 CaCO deposition OCEAN LITHOSPHERE Deep sea CaCO 3 deposition 7 11 Deep sea CaCO dissolution 3 Reservoir sizes in mol C Fluxes in 1012 mol C/yr

15 Connecting the Carbon and Alkalinity Budgets dc da = C hyd + C vol + C car r C sed = C sil a + 2C car r 2C sed da 2 dc = C sil a 2 (C hyd + C vol ) Basic Constraints of the System: Time Scales > 1 Myr τ carbon 100 kyr τ alkalinity 50 kyr Long time-scales (typically > 1 Myr): Global budgets of C and of A balanced da = 0 = C sil a = 2 (C vol + C hyd ) dc = 0

16 Basic Constraints of the System: Time Scales < 1 Myr On time scales of kyr constraint fulfilled on average only fluctuations possible classically, it has been assumed that hydrothermal and volcanic activity exhibit only small variability on these time scales C hyd + C vol = Chyd + C vol = 1 2 C sil a Hence da 2 dc = (C sil a C sil a ) da 2 dc = C sil a Sensitivity Analysis: Variable Silicate Weathering C hyd+vol 0 Constant C car r mol/yr Riverine HCO 3 supply t mol/yr CaCO 3 deposition t Full Carbon Compensation Full Alkalinity Compensation Intermediate Response Constant CO3 Response eq Alkalinity (Ocean) mol Carbon (Ocean+Atmosphere) t t ppm Atmospheric CO 2 t

17 Sensitivity Analysis: Variable Carbonate Weathering C hyd+vol 0 Constant C sil a mol/yr Riverine HCO 3 supply t mol/yr CaCO 3 deposition No compensation t Compensation for both Carbon and Alkalinity eq Alkalinity (Ocean) mol Carbon (Ocean+Atmosphere) t t ppm Atmospheric CO 2 t Sensitivity Analysis: How Does it Work in a Model? MBM Multi-Box Model of ocean-atmosphere carbon cycle ten oceanic and one atmospheric reservoirs realistic hypsometry fully coupled to copies of MEDUSA Model of Early Diagenesis in the Upper Sediment (A) bioturbated mixed-layer with 21 grid-points on top of a stack of thin layers (sediment core) solves time-dependent transport-reaction equations solids: calcite, aragonite, POM, clay solutes: CO 2, HCO 3, CO2 3, O 2 fully bi-directional exchange between the two zones

18 Bicarbonate Production Rate Scenarios 30 Carbonate Weathering HCO 3 Production 30 Silicate Weathering HCO 3 Production HCO 3 (Tmol/yr) HCO 3 (Tmol/yr) Age (kyr BP) Age (kyr BP) CO 2 Consumption Rate Scenarios 30 Carbonate Weathering CO 2 Consumption 30 Silicate Weathering CO 2 Consumption CO 2 (Tmol/yr) CO 2 (Tmol/yr) Age (kyr BP) Age (kyr BP)

19 pco 2 and Calcite Saturation Horizon Variations 360 Atmospheric CO Indo-Pacific Calcite Saturation Horizon CO 2 (ppm) Depth BSL (m) Age (kyr BP) Age (kyr BP) Summary Geochemical Carbon Cycle: complex system quantitative study requires models Four stages for development of a model 1 Identification of the problem 2 Formulation of the model 3 Resolution of the model 4 Interpretation of the results Illustration on an actual example

20 References cited T. A. Boden, G. Marland, and R. J. Andres. Global, regional, and national fossil-fuel CO 2 emissions. Data base, Carbon Dioxide Information Analysis Center (CDIAC), Oak Ridge, TN, URL T. Boden and T. J. Blasing. Record high 2010 global carbon dioxide emissions from fossil-fuel combustion and cement manufacture posted on CDIAC site. Data base, Carbon Dioxide Information Analysis Center (CDIAC), Oak Ridge, TN, URL B. P. Boudreau. Diagenetic Models and Their Implementation. Springer-Verlag, Berlin, 1997.

/ Past and Present Climate

/ Past and Present Climate MIT OpenCourseWare http://ocw.mit.edu 12.842 / 12.301 Past and Present Climate Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. The Faint Young